In a conventional system for burning flowable bio-mass material, such as wood chips, to recover energy, the chips are spread on a grate below a combustion chamber. Combustion air is blown upward through the grate and the chips, and into the combustion chamber where, with the addition of more air, combustion is completed. In common practice, the total amount of air used may vary at least from 130% to 200% of the theoretical, stoichiometric requirement. Usually as much as 60% to 80% of the total amount of combustion air is injected through the grate.
The excess air above the stoichiometric requirement is generally considered desirable to enhance drying and ensure oxidation of the chips. The increased flow of air tends to fluidize the bed to some degree, giving the chips high exposure to hot gases and radiant heat. In addition, the excess air is considered desirable to cool the grate, to prevent overheating and to extend operating life.
Several disadvantages attend this operation. The amount of excess air may cause a significant reduction in efficiency and, therefore, require a larger system, and higher fuel consumption, for equivalent power output. Further, the turbulence in the fuel bed by the combustion air entrains excessive amounts of sparks and fly ash in the combustion gases. Also, the prior operation can cause sooting or slagging problems. To remedy this condition, expensive stack gas clean-up systems are used. The excessive volume of stack gas and the added pressure drop caused by the clean-up system increase the amount of energy required for moving the gases through the system and up the stack. This increases the required operating power, and decreases system efficiency.
Accordingly, an object of the invention is to provide an improved burner or gasifier for flowable, particulate combustible material.
Another object of the invention is to provide an improved burner or gasifier in which the conditions in the fuel bed are readily controlled, for example, for better efficiency and to avoid disturbance of the bed during combustion.
Yet another object of the invention is to provide a burner or gasifier in which air flow is limited to approximately the stoichiometric requirement for combustion.
It is also an object of the invention to provide a method and apparatus for the controlled pyrolysis of particulate combustible material and which operates with high efficiency and low particulate content in the gaseous discharge.
Another object is to provide a combustion method and apparatus which attain separation and control of pyrolysis and of oxidation of combustible particulate material.
Further objects of the invention are to provide an apparatus and method characterized by improvements in the combustion of flowable particulate bio-mass fuels, such as wood chips, by improving combustion efficiency, reducing size and capital cost of equipment, lowering operating power requirements, and cleaner exhaust gases.